Hello everyone,
I am having problem with freeze-dryer, particularly with the vacuum pump.
When I start the process, the system reaches certain vacuum level (60 Pa or 0.6 mbar), hold it for couple of minutes and then it starts to rise for about 30 minutes until it reaches value of about 200 Pa (2 bar). Then it remains constant for the rest of the process.
I am new at freeze-drying procedures, but as far as I know, pressure of 200 Pa is not low enough for most of the substances. So, I don't know what to do to solve the problem. I have already checked the valves and the couplings (to prevent any leak), but it didn't help a lot. I am thinking about replacing vacuum pump, would that be helpful? Any suggestions?
Beside that, I am not really sure whether the vacuum pump should work non-stop during the freeze-drying process? In my opinion, it should stop automatically after it reaches certain level, but in my case, the pump I have is working non-stop without the rest. Is it normal for the pump not to turn off for such a long time?
Also, I don't know how to set a certain pressure level. On the freeze -dryer I am using (simple tray freeze-dryer) I can only monitor the pressure, but I don't have a possibility to influence the pressure value. Should there be such a possibility?
I am sorry for the long post and so many questions, but I am really freaking out at the moment.
If anyone could help me I would be really grateful.
Thanks in advance
Elizabeth
Hi Elizabeth,
For a general primer on Freeze Drying, look here:
https://www.spscientific.com/freeze-drying-lyophilization-basics/
The physical principle here is that pressure is proportional to the vapor pressure of the ice - or boiling point, if it's a liquid. Control the pressure, you control the ice temperature - this is regardless of the room temperature.
I'm assuming for the rest of this answer that you have a basic "tree" type lab freeze dryer, cold trap, pump and a bunch of connections to hook flasks to. If not - message back with what you have and will try and help further.
First - 0.6mbar is not that low, but shoudl be low enough to freeze dry. Ice sublimes at -29°C at 0.6mbar. Most aim for 0.2mbar so -40°C.
If the best vacuum the system can sustain, empty and dry, is 0.6mbar, you should be able to freeze dry water. Assumption number 2 here that you have a aqueous sample that is dilute, not a high concentration of sugars, lipids or solids.
It is not unusual to see the pressure rise as the sample sublimes, however a rise to 2mbar suggests either a quite large leak, or the cold trap is not managing very well. What is the cold trap temperature?
If trap is failing, then vapor will go to the pump, the pump only chugs away at liters per hour, but a few ml of liquid, when vaporised and under vacuum is thousands of liters, so if the trap doesn't collect it, the pump is choked and the pressure goes up. This can happen if when the trap is not cold enough (or can't stay cold enough under load) to condense the vapor generated. Either it's failing, or you put in a bunch of some organic highly volatile solvent that it will never catch, like ethanol.
Final thought - are your samples pre-frozen? That will help slow the vapor "rush" that you get if you load unfrozen samples.
In summary - check the cold trap, ensure the system is vacuum tight, changing the pump oil is never a bad idea, and pre-freeze. Right now, I suspect a pump oil change and that the cold trap is not 100%, but I could be wrong.
Kind regards
Rob
Elizabeth,
I have a feeling that the gas you see at 200Pa is simply water vapour.
Have a look at this.
http://www.conservationphysics.org/cool/suprcool.php
The final chart suggests that supercooled water will have a vapour pressure pretty much in line with what you see.
The initially better vacuum is trickier to explain (it rather depends on the type of pump and whether you get recondensation of the water vapour before the pump inlet.
Pumps, unless you've got a timer or some other control on them, will simply run.
There aren't many failsafe ways to stop the pump - apart from a pressure switch that might interrupt the power when you've removed all the water vapour and the pressure in the chamber ought to fall.
Rob Darrington Dear Rob, thank you so much for your answer, it was very helpful.
The cold trap is also changing its temperature during the process, and it goes from -50 to -40 (it is not constant all the time). If it turns out that cold trap is causing the problem, what can I do to solve it?
The samples I was trying to freeze dry were microbiological samples, containing microbes in nutrient medium. They were pre-frozen, but not for a very long time, so I assume I could try to freeze them deeply.
I was also suspecting vacuum leak, but then again, if it is a leak, why does it stops at 2 mbar?
Anyways, now I have few more ideas on what should I do, and I hope I will solve the problem.
Dear James, @James Garry, thank you for the help! I appreciate it.
Best regards,
Elizabeth
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